1. Field of the Invention
The invention relates to machine tools and in particular to machine tools of the kind comprising a rotatable tool head which carries a plurality of tools for performing sequential machining steps on a generally stationary workpiece in a single pass. Such machine tools are commonly known as "cat-head" machines.
Such machine tools have a number of advantages. They are able to perform rough turning and finishing operations on a workpiece in a single pass and so increase considerably the rate of production of finished workpieces. In addition, because the workpiece is usually stationary, the finished workpiece can be very rapidly removed and replaced by a fresh workpiece.
2. Review of the Prior Art
There have been various proposals for varying the relative positions of the tools and the workpiece in order to produce non-cylindrical workpieces. In general, these have comprised stationarily arranged cam surfaces arranged remote from the tool head and followed by a cam follower whose movement is translated by a mechanical linkage into relative movements see, for example, French Patent Specification No. 1,516,951 and U.S. Pat. Spec. No. 2,720,806.
It is a disadvantage of such cat-head machines that they cannot machine workpieces to complex non-uniform shapes where complex tool movements within a revolution of the cat-head are required. This is of particular significance where the workpiece is an aluminium or aluminium alloy blank for forming a piston for an internal combustion engine. It has recently become apparent that the efficiency of operation of such pistons, and of the internal combustion engines which contain them, can be improved substantially if the piston is shaped to improve its lubrication characteristics. For example, it has been found that having non-uniform surface features can produce considerable improvements in performance.